1. Field of the Invention
This invention relates generally to springs and, more particularly, to springs comprising a stack of shaped washers, such as those used in downhole drilling tools such as accelerators and jars.
2. Prior Art
In drilling tools, such as accelerators or jars, it is common to use springs to store energy between a tubular body and a mandrel slidable within the body. In prior art devices, the type of springs used as a storage device included a stack of dished washers, arranged such that compression of the dished washers resulted in storage of potential energy to be converted to kinetic energy when the springs are released. The number and arrangement of the dished washers determine the energy stored in the springs as well as the stroke supplied to the tool as the springs are released. U.S. Pat. No. 3,539,026 to Sutliff et al. and U.S. Pat. No. 4,844,157 to Taylor exhibit the use of dished washers in accelerators and jars.
There are problems associated with the use of dished washers as springs. As the washers are compressed they can become easily overstressed. In order to prevent overstressing, spacers are inserted between the washers in order to prevent complete flattening under pressure. Such spacers are shown in FIGS. 2 and 3 herein. The use of spacers not only increases the amount of material and number of components of the springs, but also increases the overall length of the spring assembly by limiting the stroke associated with each combination of washers. Also, as dished washers are compressed, friction from the facing surfaces rubbing together becomes a factor in determining how much energy is required to compress the springs and how much potential energy is stored in the compressed springs. Another problem which exists in the use of dished washer springs results from the washers sliding off center before compression, causing uneven loading of the stack of washers.